There has been confusion around the Windows Vista Versions available and where each fit in the market space, and on you network.  To that extent there are 5 versions of Windows Vista being released in the US, however some may not be applicable to your environment:

Business Versions:
Windows Vista Business – the operating system designed for organizations of all sizes. Windows Vista Business will help keep PCs running smoothly and more securely and will provide dramatic new infrastructure improvements that will enable IT staff to spend less time focused on day-to-day maintenance of PCs and more time on adding strategic value to the organization. These are some of the specific features of Windows Vista Business:
A new user interface, named Windows Aero™, is designed to deliver the most productive, highest-performing desktop experience possible. 
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Windows Vista Business makes it easier than ever to manage huge volumes of business documents by integrating search throughout the operating system and providing new ways to organize files.
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Windows Tablet PC technology is built in to Windows Vista Business, along with improved deployment and management capabilities.

Windows Vista Enterprise – (Available only to customers who have PCs covered by Microsoft Software Assurance or a Campus or School Agreement) In addition to all the features available in Windows Vista Business, Windows Vista Enterprise is designed to provide higher levels of data protection using hardware-based encryption technology, improve application compatibility and will enable organizations to standardize on a single deployment image with the inclusion of all Windows user-interface languages. The following features are exclusive to Windows Vista Enterprise.
Windows BitLocker™ Drive Encryption helps prevent sensitive data and intellectual property from falling into the wrong hands if a computer is lost or stolen.
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Virtual PC Express is one of several built-in tools that improve application compatibility with previous versions of Microsoft operating systems. Virtual PC Express enables a legacy application to run unchanged on a legacy Windows operating system in a virtual environment on top of Windows Vista Enterprise.
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Subsystem for UNIX-based Applications enables users to run UNIX applications unchanged on a Windows Vista Enterprise-based PC.

Consumer Versions:
Windows Vista Home Basic – People with basic computing needs. For consumers who want to simply use the PC to browse the Internet, correspond with friends and family through e-mail or perform basic document creation and editing tasks, Windows Vista Home Basic will deliver a safer, more reliable and more productive computing environment.

Windows Vista Home Premium – Designed for consumers who use mobile or desktop PC functionality more effectively while enabling the enjoyment of new, exciting digital entertainment experiences. Windows Vista Home Premium improves every aspect of digital entertainment experiences, including photos, video, TV, movies, music and games. Windows Vista Home Premium includes everything in Windows Vista Home Basic, along with additional features and enhancements such as the following:
The new user interface Windows Aero is in this edition of Windows Vista.
Windows Media Center - turn the PC into an all-in-one home entertainment center. Consumers can use Media Center to record and watch TV shows and access new kinds of online entertainment content. It also provides the ability to connect Windows Vista Home Premium to Xbox 360™, extending the Media Center experience to multiple rooms.
Windows Tablet PC technology, 
Integrated DVD burning and authoring allows users to seamlessly burn personal videos photos and files to video or data DVDs, and easily create professional-looking DVDs from home movies that can be shared with family and friends.

Windows Vista Ultimate – The edition of Windows Vista that has it all. It brings together all the entertainment features, mobility features and business-oriented features available in Windows Vista.  The consumer features are not manageable via GPO, and Windows Vista Ultimate will not be deployable via Volume Licensing Keys.  However, this version can join a domain and use GPO to manage the business features.

Microsoft is on schedule to complete the Windows Vista product this year, with business availability in November 2006 via the Volume Licensing Channels.  The broad consumer availability will be in January 2007.  If you are looking to purchase new PCs between now and the end of the year, please work with your OEM on Windows Vista capable systems.